Controlled temperature rocket nozzle

ABSTRACT

A rocket nozzle has axially double bell shape, i.e. is of so-called “Dual Bell” type, and has an outwardly directed change of curvature of the contour line or generatrix at the inflection point between the two bell shapes. For obtaining an improved cooling action on the nozzle wall the change of curvature amounts to between 2° and 7°, said inflection point (I) being located between a location at the area ratio ε=10 and a location at 0.85×ε max  of the nozzle.

[0001] The invention refers to a rocket nozzle having axially doublebell shape, i.e. of so-called “Dual Bell” type, and having an outwardlydirected change of curvature of the contour line or generatrix at theinflection point between the two bell shapes.

[0002] The Dual Bell shape of rocket nozzles is known from the early60's for providing an altitude compensation. In sea level operation modesuch a Dual Bell nozzle the inflection point will force the flow toseparate from the nozzle wall at the desired location, thus increasingsea level thrust. In altitude operation mode the plume gradually expandsuntil it finally attaches to the nozzle wall downstream of theinflection point. In reality, however, the Dual Bell nozzle concept hasseveral inherent inefficiencies which reduce its performance from thetheoretical optimum.

[0003] On the other hand, the function of the rocket nozzle is to expandand accelerate the gas to high velocity and thereby give thrustefficiency and payload capacity. The thrust efficiency is especiallyimportant to upper rocket stages. High thrust performance means highwall temperatures and as a consequence leads to exotic and expensivetechnologies. The temperature of the walls of a rocket nozzle isdependent on the pressure at the wall and the speed of the flow at thewall.

[0004] For controlling the wall temperature of a rocket nozzle,particularly wall portions which are not actively cooled by convectioncooling, several techniques have been suggested. First of all, thematerials used are to have strength at very high temperatures which ofcourse is expensive. The nozzle walls also may be covered by coatingsthat insulate and allow high surface temperatures. This is alsoexpensive. Finally, a cooling film might be used in combination with acontinuous nozzle contour.

[0005] In the case of using metallic materials, such materials have highcost and a nozzle structure must be built with many joints due to thematerial availability. The large number of joints, however, lower thereliability. Alternatively, a ceramic matrix composite material may beused. In this case the cost is very high and the reliability might bequestioned due to little experience for application in rocket nozzles.

[0006] Thus, coatings add cost and the potential to lower the steadystate temperature is limited. A coating also means reduced reliabilitydue to increased complexity. As to the case of film cooling, there isnormally no gas to produce film available for closed cycle engines.Tapping of gas for film cooling purpose would mean serious performanceloss.

[0007] It has now turned out that a simple and inexpensive way to obtaina control of the temperature of the nozzle walls might be obtained basedon the Dual Bell shape but adapted as suggested according to the presentinvention. The invention thus is substantially distinguished in that forobtaining an improved cooling action on the nozzle wall the change ofcurvature amounts to between 2° and 7°, said inflection point (I) beinglocated between a location at the area ratio ε=10 and a location at0.85×ε_(max) of the nozzle.

[0008] By introduction of a discontinuity in the meridional plane for anozzle contour the wall temperature will be lowered faster than whatwould be the case for the normal continuous contour. The temperature ofthe nozzle wall from the point of the discontinuity is made close toconstant. The temperature that decides the nozzle material therefore islowered. As a side effect, by introduction of a discontinuity thebehaviour of a cooling film could be controlled. At the inflection pointthe film close to the nozzle wall will be subjected to a suddenacceleration just downstream of said discontinuity which will stabilisethe film and prevent mixing. The efficiency of the film is thenmaintained.

[0009] The invention will be further described below by way of examplewith reference to the accompanying drawings in which

[0010]FIG. 1 is a longitudinal section of a nozzle shape according tothe present invention, the left half of which illustrates a location ofthe inflection point I at a location ε=10 while the right halfillustrates a location of the inflection point I at ε=85 percent ofε_(max), and

[0011]FIG. 2 shows the curve of the relation between wall temperatureand axial length of the nozzle according to the invention.

[0012] In FIG. 1 it is thus illustrated a rocket nozzle 1 of so-called“Dual Bell” type, i.e. having an axially double bell shape. Similar towell-known altitude compensating nozzle structures there is aninflection point I on the contour line or generatrix where there is asudden change in the curvature of said contour line, in other wordswhere the upper bell shape changes to the further bell shape nextthereto. Unlike the known Dual Bell structures where the change ofcurvature amounts to at least 9° in order to provide for a suddendirection change for obtaining the desired separation of the flow alongthe nozzle wall at said point, the present invention suggests that thechange of curvature amounts to only between 2 and 7°, the inflectionpoint I being located between a location at the area ratio ε=10 and thelocation at 0.85×ε_(max) of the nozzle. ε is the area ratio whichamounts to ε=1 at the throat of the nozzle.

[0013] According to the invention the inflection point might be locatedat any suitable location between the two stipulated limits stated above.

[0014] The sudden acceleration of the film flow along the wall caused bythe change of curvature of the wall contour line provides for animproved cooling effect starting immediately downstream said inflectionpoint and maintaining said effect to such an extent that the rest of thewall downstream said inflection point will be kept almost constant. Saidreduced wall temperature thus allows the use of a wall material not atall as temperature resistant as requested in prior art and consequentlya cheaper structure.

1. A rocket nozzle having axially double bell shape, i.e. of so-called “Dual Bell” type, and having an outwardly directed change of curvature of the contour line or generatrix at the inflection point between the two bell shapes, characterized in that for obtaining an improved cooling action on the nozzle wall the change of curvature amounts to between 2° and 7°, said inflection point (I) being located between a location at the area ratio ε=10 and a location at 0.85×ε_(max) of the nozzle. 